The present embodiments relate to a method for creating a therapy plan for a particle therapy and a filter apparatus for a particle therapy system.
During a particle therapy treatment (e.g., of cancerous diseases), a particle beam made up of protons or heavy ions (e.g., carbon ions) is generated in a suitable accelerator. The particle beam is guided into a treatment room using beam control and enters the treatment room by way of an exit window. The particle beam may alternately be directed into different treatment rooms by an accelerator. In the treatment room, a patient to be treated is positioned on a patient couch, for example, and may be immobilized using an immobilization device.
The irradiation of a target area (e.g., a tumor, tissue) may take place layer by layer. As a function of the energy of the particle beam, the particle beam reaches different depths in the tissue, so that the tissue can be subdivided into disk-type sections or layers of the same depth of penetration. The focused particle beam is moved across the individual layers of the target area (e.g., “beam scanning”), so that several points within a layer, which lie on a grid for example, are irradiated. As the radiation intensity and/or energies are selected correctly, regions with a complicated structure can also be irradiated accurately. The arrangement of the layers and points to be irradiated is selected such that the planned dose distribution can be achieved.
During irradiation with an accelerator, minimal energy, which the accelerator is to provide with respect to a good beam quality, is needed. This minimal energy corresponds to a water equivalent thickness of 20 mm, for example (i.e., the beam reaches 20 mm into the tissue). As a result of the minimal energy required, the active energy modulation of the accelerator enables target areas, which are located at 20 mm or lower below the skin surface, to be treated. To reach the tissue between the skin and 20 mm below the skin surface, a passive filter (e.g., a “range shifter”) is provided, which decelerates the particle beam so that the irradiation of tumors close to the skin is enabled. Such a filter is generally made from a water-equivalent material such as, for example, PMMA. The filter, generally made in the manner of a plate, is held between the beam exit and the patient and has an intensity which corresponds to the 20 mm water equivalence, for example. A deceleration of the beam energy within the filter plate therefore takes place, and the irradiation of a body region to be treated immediately behind the filter plate is enabled.
Since scattering effects take place as a result of the interaction between the particle beam and the atoms of the filter plate, the filter plate is positioned as close as possible to the patient. Furthermore, the orthogonality of the filter plate relative to the particle beam must be ensured.
With current accelerators, the filter is usually adjustably arranged in the region of the exit window, and when removing the filter for the therapy, the filter plate often collides with the patient couch.
To treat a patient with the particle beam, a therapy plan is created beforehand. The therapy plan defines which layers of the target area (e.g., the tumor) are to be radiated with which dose and from which direction. The aim is to irradiate the tumor as efficiently as possible, with organs at risk (e.g., an optic nerve or parts of the brain) being excluded from the irradiation or the applied dose in the healthy tissue of the “irradiation channel” being minimized.
With current therapy planning systems, the “range shifter”, which is a rigid plate with a constant water-equivalent thickness, is provided in the therapy plan. The real thickness, quality and homogeneity of the plate material are not measured by the planning system. Instead, prior to using the filter plate for the first time, the properties of the filter plate are determined in a complicated process and used as constant variables in subsequent therapies.